There are many good oxidation and corrosion resistant coatings used in industry for various applications and for use in various environments. Articles composed of iron-, cobalt-, or nickel-based superalloys have been developed for use in applications, such as aerospace applications, and for use as blades, vanes, seals and other components utilized in gas turbine engines. In these applications, it is important that the articles have sufficient protection against undue oxidation and sulfidation since such corrosion can affect the useful life of the article resulting in reduced performance and possible safety problems. Although various superalloys have a high degree of corrosion resistance, such resistance decreases when the superalloys are operated in or exposed to high temperature environments.
To increase the useful life of components made of alloys and superalloys, various coatings have been developed. Aluminide coatings were initially used to provide a corrosion resistant outer layer but such layer was observed to crack when subjected to mechanically or thermally induced strain. Another class of coatings developed was the MCrAlY overlay coatings where M represents a transition metal element such as iron, cobalt or nickel. The coatings have been found to be more effective than the aluminide coatings in extending the useful life of alloy components in high temperature environments.
A current problem with conventional MCrAlY coatings on superalloy substrates is interdiffusion of coating elements into the substrate and substrate elements into the coating after long times of high temperature exposure. Certain substrate elements like titanium have been found to diffuse through the MCrAlY coating to the external surface oxide scale and to make said oxide scale less protective. It would be desirable to modify current MCrAlY coatings to reduce this interdiffusion effect.
Although MCrAlY has overall been a successful class of coatings having good oxidation and corrosion resistance for superalloys, improvements have been made to the MCrAlY coatings.
U.S. Pat. No. 3,993,454 discloses coatings which are particularly suited for the protection of nickel and cobalt superalloy articles at elevated temperatures. The protective nature of the coatings is due to the formation of an alumina layer on the surface of the coating which serves to reduce oxidation/corrosion. The coatings contain aluminum, chromium, and one metal chosen from the group consisting of nickel and cobalt or mixtures thereof. The coatings further contain a small controlled percentage of hafnium which serves to greatly improve the adherence and durability of the protective alumina film on the surface of the coating. U.S. Pat. No. 4,585,481 discloses a similar coating except that yttrium and hafnium are used together along with silicon.
It is object of the present invention is to provide thermal spray process for producing a MCrAlY- based coating having a carbide dispersion throughout the coating.
Another object of the present invention to provide a process for producing a MCrAlY-based coating using a detonation gun and wherein the coating has a carbide dispersion throughout.
Another object of the present invention is to provide a MCrAlY-based coating wherein the chromium component is substantially in the elemental chromium form with a minor amount in the chromium carbide form.